Method for particle size distribution control in a bayer circuit decomposition chain, comprising an agglomeration phase

ABSTRACT

In a BAYER circuit, a process for controlling precipitation in which particle size quality of alumina hydrate produced in the circuit and circulating in feed tanks is monitored utilizing a calibration step and a control step. In the calibration step, cumulative percentage of alumina hydrate particles circulating in the feed tanks in the circuit that are finer than X2 μm, defined as CPFT X2, is measured vs. time and cumulative percentage of alumina hydrate particles circulating in the feed tanks in the circuit that are finer that X1 μm, defined as CPFT X1 vs. time, is measured, where X1 and X2 are predetermined particles sizes and X1 is smaller than X2. A relationship R between CPFT X1 and later changes in CPFT X2, is determined and upper and lower trigger thresholds of CPFT X1 which correspond to maximum permissible variation in CPFT X2 are defined. In the control step, CPFT X1 and CPFT X2 are regularly measured, and R and the correlation between CPFT X2 and the particle size of hydrate produced are updated. Corrective action is taken at the beginning of precipitation when the measured value of CPFT X1 reaches one of the trigger thresholds.

FIELD OF THE INVENTION

The invention relates to the precipitation of alumina trihydrateaccording to the Bayer process, carried out in an American typeprecipitation system including a preliminary agglomeration phase.

DESCRIPTION OF RELATED ART

The Bayer process can produce alumina from bauxite ore, particularlyalumina designed to be transformed into aluminum by igneouselectrolysis. According to this process, the bauxite ore is treated whenhot by means of an aqueous sodium hydroxide solution with an appropriateconcentration in order to obtain a suspension containing pregnant sodiumaluminate liquor and insoluble residues. After separation of theseresidues, the pregnant sodium aluminate liquor, also called Bayerliquor, is decomposed by seeding with recycled aluminum trihydroxideparticles until aluminum trihydroxide grains (also called aluminatrihydrate or hydrargillite) are obtained, and which are themselves thencalcinated to obtain an alumina with particular particle sizedistribution and physicochemical properties. The sodium aluminate liquordepleted in alumina (spent liquor) is then recycled to digest the ore,possibly after being concentrated, by the evaporation and addition ofsodium hydroxide or caustic soda.

The productivity of the liquor during its crystallisation is defined bythe quantity of alumina restored in the form of alumina trihydrate bycrystallisation of the pregnant liquor, related to a given volume ofpregnant liquor. The productivity expressed in kilograms of alumina percubic meter of liquor (kg of Al₂O₃/m³), is related to the causticconcentration in the pregnant liquor. In general, this concentration inAmerican type Bayer processes is close to 100-130 g of Na2O/liter, whichis lower than in European type Bayer processes, and this explains why aproductivity in the crystallisation of the pregnant liquor is consideredto be good when it exceeds 70 kg of Al₂O₃/m³ for an American type Bayerprocess, or when it exceeds 80 kg of Al₂O₃/m³ for a European type Bayerprocess.

The difference between European type and American type Bayer processesis in the solid content of the slurry during precipitation. The slurryis the result of introducing a recycled alumina trihydrate seed into thealuminate liquor, and part of the alumina in solution changing to thesolid phase. We will define the solid content in the slurry as theweight of solid particles present in the slurry per unit volume ofpregnant aluminate liquor entering into the precipitation workshop (andnot per unit volume of the suspension).

Alumina to be transformed into aluminum by igneous electrolysis musthave a number of properties, including:

-   -   good flowability so that electrolysis tanks can be continuously        supplied with controlled quantities of alumina,    -   a high dissolution rate,    -   a low tendency to dusting.

These properties are closely related to the morphology and the particlesize distribution of alumina grains, themselves closely related to themorphology and particle size distribution of hydrargillite particlesformed during the precipitation. It is particularly important to limitthe proportion of very fine particles that can be classified in two maincategories: fines (for which the average diameter is between 10 and 50μm) and ultrafines, for which the average diameter is less than 10 μm.Since a good correlation is observed between the particle sizedistribution of alumina and the particle size distribution of theproduction hydrate from which it is derived, an attempt is made tocontrol the particle size of the hydrate circulating in theprecipitation series, and particularly in the crystal growth phase. Thusfor example, in order to obtain a good quality “metallurgical” alumina,an attempt is made to obtain a circulating hydrate for which the amountpassing a 45 μm sieve is less than 10%, i.e. a suspension containingless than 10% of particles with a diameter of less than 45 μm. In therest of this discussion, we will denote the quantity passing Xmicrometers as “%<X”.

Concerning the American type Bayer process, the precipitation comprisesa preliminary agglomeration phase characterized by a particularly lowsolid content. In patent U.S. Pat. No. 4,234,559, the precipitationcircuit comprises firstly a series of agglomeration tanks and then aseries of feed tanks and finally three classification tanks (primary,secondary, tertiary). While the hydrate produced is derived from theunderflow from the primary classification tank, the fines seedoriginating from the underflow from the tertiary classification tank isinserted in controlled quantities into the series of agglomeration tanksand the larger seed originating from the underflow from the secondaryclassification tank is added into the series of feed tanks. Since thefines are destroyed during the agglomeration phase, the problem ofcontrolling the particle size of the hydrate produced does not arise.

But attempts are still being made to increase the productivity of theAmerican type Bayer process, which is lower than the productivity ofEuropean type Bayer cycles. In U.S. Pat. No. 5,158,577 and EP 0 515 407,only part of the pregnant liquor is added into the agglomeration tankseries, and the rest is added directly to the crystal growth. This canresult in different solid contents in these two parts of thecrystallization system, a low solids content in agglomeration tankswhich is essential if it is required that agglomeration should takeplace under good conditions, and a high solid content in feed tankswhich can increase productivity.

But an instability in the precipitation is observed if the solid contentis increased at the feed and if the number of agglomeration tanks islimited, with a serious risk of the sudden appearance of largequantities of fines in the circulating hydrate (particle size crisis).This type of crisis should be avoided, since if no correction is made,the particle size quality of the produced hydrate is stronglydeteriorated.

The particle size instability is due to a reduction in the ratio betweenthe sum of the production hydrate and the fine seed sent toagglomeration, and the circulating hydrate. A reduction in this ratiomakes it impossible to implement effective corrective actions when adrift of the amount passing 45 μm (%<45) is observed on the circulatinghydrate.

SUMMARY OF THE INVENTION

Therefore, the applicant attempted to define a process that couldincrease the productivity of the American type Bayer process bypreventing unacceptable particle size fluctuations, particularly thesudden appearance of large quantities of fines and ultrafines in thecirculating hydrate.

The process developed by the applicant is a process for controlling theprecipitation of an American type Bayer circuit including a preliminaryagglomeration phase, a crystal growth phase and a classification phase,in which the particle size quality of the hydrate produced is monitoredby measuring the amount of rotating hydrate passing X2 μm in feed tanks,characterized in that it comprises:

-   -   a) a preparation step carried out once and for all, intended        firstly to setup a relation R in intensity and in time between        circulating hydrate material passing X1 μm and material passing        X2 μm, where X1 is less than X2, and secondly to define trigger        thresholds on the value of material passing X1 μm, starting from        the maximum authorized variation interval on values passing X2        μm;    -   b) control of the process itself, carried out during the        installation operating period which, apart from the daily        measurement of material passing X2 μm and a regular update of        the correlation between the said material passing X2 μm and the        particle size of the hydrate produced, a daily measurement of        the circulating hydrate passing X1 μm and a regular update of        the relation R between the said material passing X1 μm and the        said material passing X2 μm, and triggering of corrective action        in the slurry at the beginning of the precipitation when the        measured value of material passing X1 μm reaches one of the        regularly updated trigger thresholds determined in the previous        step.

This corrective action in the slurry at the beginning of precipitationmay be a modification to the temperature of the aluminate liquor addedinto the agglomeration tank, the addition of additives at the beginningof the crystal growth system such as “Crystal Growth Modifiers”described in U.S. Pat. No. 4,737,352, recycling a part of the end ofcrystallization slurry, or preferably, modification of the solid contentin the slurry in the first agglomeration tank.

The solid content in the slurry in the agglomeration phase may bemodified simply by adding more or less pregnant aluminate liquor in thefirst agglomeration tank, the remaining aliquot being directed to thefeed tank. If the quantity of material passing X1 μm is too great, thereare too many fines; the amount of pregnant aluminate liquor fed into theagglomeration tank must be increased. If the amount passing X1 μm is toolow, there is a risk that productivity will drop; the pregnant aluminateliquor feed from the agglomeration tank must be reduced.

The preliminary step, carried out once only, is intended to determinethe relation R and trigger thresholds for the values of X1 μm that willbe used at the beginning of application of the process control. Thispreliminary step comprises the following steps:

-   -   a1) Daily measurement of material passing X1 μm in the slurry at        a particular point in the precipitation system, which is used to        produce a first particle size time diagram represented by a        curve Y=%<X1(t).    -   a2) Daily measurement of material passing X2 μm in the slurry at        a particular point in the precipitation system, which is used to        produce a second particle size time diagram represented by a        curve Y=%<X2(t)and in which X2, greater than X1, is a value        already known for its good correlation with the particle size of        the hydrate produced. For example, it may be material passing 45        μm measured in slurry at pump-off.    -   a3) Creation of an empirical relation between the two particle        size time diagrams, the purpose of which is to characterize the        relation between the variation of the population of material        passing X1 μm and the variation of the population of material        passing X2 μm, in intensity and in time. This relation R may be        written in the form:        R(%<X2(t), %<X1(t−τ))=0        where t is the date on which material passing X2 μm is measured        and τ is a characteristic time interval estimated by observing        the occurrence of the same accidental phenomenon on each curve        (the same type of extreme, an inflection, etc.).    -   a4) Definition of the maximum threshold and the minimum        threshold of material passing X1 μm, obtained from the relation        R previously established and a maximum interval of the        authorized variation of values of material passing X2 μm.

The purpose of the relation R is to predict the variation in theparticle size, in other words to anticipate crises, by observing thevariation in the population of the finest hydrate particles (which havea size of less than X1 μm). The applicant observed that material passingX1 μm enables anticipating a change in the material passing a higher X2value; an accident on the time diagram %<X1(t) is amplified and shiftedin time on the %<X2(t) time diagram. The time shift “τ” is higher as thedifference between X1 and X2 increases. In practice, a value X2 greaterthan 40 μm (normally 45 μm) will be chosen, and the value of X1 will betaken to be less than or equal to 20 μm.

The measurement point is preferably at pump-off, but it may take placeearlier, provided that it remains within the crystal growth series.Measurement points for material passing X1 and for material passing X2may be different. However, they must remain the same throughout theprocess control and must be as far as possible from the points at whichdisturbing additions are made irregularly in the slurry.

Measurements referred to as being “daily” are regular measurements andalthough they are not necessarily daily, they are sufficient frequent togive useable time diagrams.

Concerning the required variation interval of material passing X2 μm, amaximum is defined above which it is known that the particle size of thehydrate produced is no longer satisfactory (too many fines) and aminimum is defined below which it is known that economic operatingconditions become bad.

The trigger thresholds are thus determined from the time diagram for thematerial passing X1 μm, taking account not only of the maximumauthorized variation interval on the values of material passing X2 μm,but also the uncertainty of the measurement of material passing X1 μmand the stability of the efficiency of the hydrate classificationsystem.

This preparatory step, at the end of which the time diagrams, therelation R and the trigger threshold become operational, is of the orderof three months. But this step can be accelerated either by deliberatelytriggering an excess creation of fines, or by analyzing previousparticle size results if they contain the required information aboutmaterial passing X1 and X2 μm.

Since precipitation is a complex phenomenon that depends on a largenumber of parameters (particularly the composition of the treatedbauxite that may change in time), instead of using a relation Restablished once and for all, it is better to use a relation that isregularly updated. Similarly, the correlation between the value ofmaterial passing X2 μm and the particle size of the hydrate producedmust be updated regularly. This regular updating may be applied at aless frequent rate than daily measurements (for example monthly).

The actual process control comprises the following phases:

-   -   b1) A daily measurement of the material passing X1 μm in the        slurry at a particular point in the precipitation system, in        order to complete the first particle size time diagram        represented by the curve Y=%<X1(t).    -   b2) A daily measurement of the material passing X2 μm in the        slurry at a particular point in the precipitation system, in        order to complete the first particle size time diagram        represented by the curve Y=%<X2(t).    -   b3) Regular updating(for example monthly) of the empirical        relation R between the two particle size time diagrams and the        definition of trigger thresholds of material passing X1 μm, or        updating after an important modification in a process parameter.    -   b4) Triggering of a corrective action in the slurry at the        beginning of the precipitation when the measured value of        material passing X1 μm reaches one of the thresholds defined in        b3).

This corrective action is preferably a modification to the solid contentin the slurry in the first agglomeration tank. If the maximum thresholdis reached, there are too many fines, and the solid content is reducedby introducing a stronger aliquot of the pregnant aluminate liquor intothe first agglomeration tank. If the minimum threshold is reached, thefeed of pregnant aluminate liquor into the agglomeration tank is reducedand the feed into feed tanks is increased.

As for the preparatory step, empirical relations are determined startingfrom daily particle size measurements made on the circulating hydrate.As for the preparatory step, the daily rate is not necessarily daily,but is sufficiently frequent to be able to produce useable timediagrams. Trigger thresholds on the “X1 μm passing material” curve arealso deduced from the particle size time diagram; they take account ofthe uncertainty of the measurement of material passing X1, the maximumauthorized variation interval on values of material passing X2 μm, andthe stability of the efficiency of the hydrate classification system.

For relatively low solid content (≈350 g/l aluminate), it is sufficientto measure amount passing X1=20 μm. This may be done using instrumentsthat diffract laser beams and make mass determinations (MalvernMastersizer, Cilas, etc.).

But in order to increase productivity, much higher solid content must beachieved, comparable to those achieved in European type processes, inother words greater than 700 g/l aluminate. However, as the solidcontent in feed tanks increases, the need for a low value of X1 alsoincreases. When the solid content is high, a longer time is necessary tocorrect particle size disturbances; therefore, it is necessary to have agreater time shift τ; and this time shift is proportional to thedifference between X1 and X2.

In this case, it is preferable to measure the amount passing 10 μm, oreven a lower value (down to 1.5 μm). When X1 becomes this low, itbecomes necessary to use a celloscopic measurement (COULTER or ELZONE)that determines counts rather than mass. These measurements are moredifficult, but the economic consequences are negligible.

Productivity can also be increased by increasing the caustic content ofthe aluminate liquor. The applicant has observed that this method ofcontrolling the particle size can be applied very well on industrialinstallations with concentrations reaching 160 g of Na2O/liter.

Concerning the corrective action, it is also recommended that anempirical relation should be set up in advance to quantify the effectsof the said action, for example during the preparatory phase. Thus, inorder to quantify the effects of the modification to the solid contentin the first agglomeration tank, the first step will be to establish theactual relation between the said solid content and the proportion ofdestroyed fines.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a prior art Bayer circuit; and

FIG. 2 is a schematic diagram of a Bayer circuit according to theinvention.

EMBODIMENTS OF THE INVENTION EXAMPLES

The embodiment of the invention will be better understood from thefollowing description of a number of examples.

FIGS. 1 and 2 show the part of the Bayer circuit corresponding to theprecipitation phase, the precipitation being of the American type with apreliminary agglomeration step.

FIG. 1 illustrates the first example typical of prior art.

FIG. 2 illustrates subsequent examples describing two embodiments ofthis invention.

These embodiments are described in examples 2 and 3. To simplify thepresentation, we have illustrated the classification phase with allconventional classification devices in example 1. In fact, andparticularly with the increase in the solid content, one of ordinaryskill in the art could select additional or replacement systems such ascyclones or filters.

Example 1

(Prior Art)

The pregnant aluminate liquor 1 enters the precipitation circuit at atemperature of about 75° C., enters the first agglomeration tank A. Ithas a caustic concentration of 130 g of Na2O/liter.

The tertiary seed 9, composed essentially of fine and ultrafineparticles, is added into the first agglomeration tank A and is mixedwith pregnant liquor. The resulting slurry 2 passes through a sequenceof agglomeration tanks such that practically all fines and ultrafineshave disappeared after an average residence time of about 5 hours.During the agglomeration phase, the solid content in the slurry hasincreased from 20 g/l aluminate to about 30 g/l aluminate.

At the exit from the agglomeration series, the slurry 3 is added intothe first feed tank N with the secondary seed 6, and the additionincreases the solid content by 80 g/l aluminate. The new slurry 4 passesthrough a sequence of feed tanks with an average residence time of about20 hours, and cooling by about 5° C. At the exit from the feed, theslurry 5, called the “pump-off”, has a solid content of about 175 g/laluminate. By measuring the amount of this slurry 5 passing 45 μm at theexit from the crystal growth M1, the particle size quality of theproduction hydrate can be estimated so that an optimum variationinterval can be defined for this material passing 45 μm. Under theconditions in the example, approximately 10% of material passing 45 μmwill give a good quality “metallurgical” alumina.

The slurry 5 at the exit from the crystal growth is then added into thefirst classifier tank PT. The underflow 100 from the first classifier PTproduces production hydrate and the overflow 6 that has a solid contentof 100 g/l aluminate, is added into a second classifier tank ST. Theunderflow from the second classifier tank ST acts as a secondary seed 8that is reinjected at the beginning of the crystal growth N and theoverflow 7, which then has a solid content of only 20 g/liter, is sentto a third classifier tank TT. The overflow 10 from the third classifiertank is the spent aluminum liquor that is reinjected as the bauxitegreen liquor at the beginning of the Bayer cycle, after concentration byevaporation and the addition of caustic soda. The overflow 9 from thethird classifier tank is added back as the tertiary seed into the firstagglomeration tank A after filtration and washing.

Example 2

Precipitation According to the Invention with an Average Solid Contentin the Circulating Hydrate

The pregnant aluminate liquor 1 that enters the precipitation circuit ata temperature of about 75° C., is separated into two aliquots, the firstaliquot 1 a representing about one third of the total liquor being addedinto the first agglomeration tank A, and the second aliquot 1 n beingadded into the first feed tank N. The caustic concentration in thepregnant aluminate liquor 1 is 130 g of Na₂O/liter.

The tertiary seed 9 a, composed essentially of fine and ultrafineparticles, is added into a first agglomeration tank A after filtrationand partial or complete washing, and is mixed with the pregnant liquor.The resulting suspension 2 follows a sequence of agglomerating tankssuch that a predefined proportion of fines and ultrafines hasdisappeared after an average residence time of 5 hours. During theagglomeration phase, the quantity of hydrate is increased by 10 g/laluminate entering the workshop (1 a and 1 n).

The slurry 3 at the exit from the agglomeration series is added into thefirst feed tank N with the second aliquot 1 n of pregnant aluminateliquor, the rest of the tertiary seed 9 n, washed or unwashed, and thesecondary seed 8, the addition of which increases the solid content by220 g/l aluminate. The new slurry 4 follows a series of feed tanks withan average residence time of 20 hours and cooling of 5° C. At the exitfrom the crystal growth, the slurry 5 has a solid content of about 350g/l alumniate.

The slurry 5 at the exit from the crystal growth is added into a firstclassifier tank PT. The underflow 100 from the first classifier PTsupplies the production hydrate, and the overflow 6 which has a solidcontent of 250 g/l aluminate is added into a second classifier tank ST.The underflow from the second classifier tank ST acts as a secondaryseed 8 that is reinjected at the beginning of the crystal growth N andthe overflow 7, which then only has a solid content of 30 g/liter, issent to the third classifier tank TT.

A measurement M1 of the material passing 45 μm, and a measurement M2 ofthe material passing 20 μm, are made in the slurry every day atpump-off, using a laser diffraction apparatus.

Observations of the particle size quality of the hydrate produced can beused to define the authorized variation interval on values of materialpassing 45 μm, and the target set value C on the material passing 20 μmis defined, by means of the empirical relation defined during thepreparation phase and continually updated afterwards.

Depending on the difference between M2 and the value of the set value Cdefined in advance to guarantee the particle size quality of the productand the differences M2−C obtained in the previous days, the quantity offines to be agglomerated in addition to or less than the previous day,are determined.

During the preparation phase, the relation between the solid content inthe agglomeration phase and the proportion of destroyed fines wasdetermined. This relation is used to fix the aliquot 1 a used in theagglomeration. This aliquot 1 a fixes the solid content in theagglomeration tanks and therefore the change required for destruction ofthe fines.

Furthermore, starting from measurements M1 and M2 made on previous days,the relation between the set value C and the required level M1 for thematerial passing 45 μm, and secondly the relation between the solidcontent and fines destroyed in the agglomeration, are adjusted.

Example 3

Precipitation According to the Invention with a High Solid Content inthe Circulating Hydrate

The pregnant aluminate liquor 1 arriving in the precipitation circuit ata temperature of about 75° C., is separated in two aliquots, the first 1a representing about half of the total liquor being added into the firstagglomeration tank A, the second in being added into the first feed tankN. The caustic concentration in the pregnant liquor 1 a is 130 g ofNa₂O/liter.

The filtered tertiary seed 9, composed essentially of fine and ultrafineparticles, is added into the agglomeration step A and mixed with thefraction 1 a of pregnant liquor. The resulting slurry 2 stays in theagglomeration phase for about 5 hours. The proportion between thequantity of tertiary seed and the flow of liquor 1 a is adjusted suchthat a predefined proportion of fines and ultrafines disappears. Thequantity of hydrate during the agglomeration phase is increased by 15g/l aluminate entering in the workshop (1 a and 1 n).

At the exit from the agglomeration series, the slurry 3 is added intothe first feed tank N and with the second aliquot 1 n of pregnantaluminate liquor and the secondary seed 8, the addition of whichincreases he solid content by 840 g/l aluminate. The new slurry 4 passesthrough a series of feed tanks with an average residence time of 18hours and cooling by 10° C. At the exit from the crystal growth, theslurry 5 has a solid content of about 1000 g/l aluminate.

At least one measurement M1 of the material passing 45 μm is made everyday at this location using a laser diffraction apparatus, and ameasurement M2 of the material passing 1.5 μm is made with an ELZONEcounter.

The slurry 5 at the exit from the crystal growth is added into a firstclassifier tank PT. The underflow 100 from the first classifier PTsupplies the production hydrate and the overflow 6, that has a solidcontent of 870 g/l aluminate, is added into a second classifier tank ST.The underflow from the second classifier tank ST is used as a secondaryseed 8 that is reinjected at the beginning of the crystal growth N afterfiltration and the overflow 7, that has a solid content that no longerexceeds 30 g/liter, is sent to the third classifier tank TT.

The process for monitoring and controlling the particle size of thehydrate in the slurry at pump-off is identical to that described inexample 2.

The process according to the invention has the following advantages:

-   -   particle size fluctuations related to the increase in the        inertia of the system originating from the increase in solid        contents are avoided.    -   The productivity of liquors can thus be increased:        -   by controlling crystallization at high solid contents            without endangering the quality of the alumina produced,        -   by adjusting the pump-off particle size to the maximum level            compatible with quality requirements and the production            classification system.

1. In a BAYER circuit including a preliminary agglomeration phase, a crystal growth phase and a classification phase, a process for controlling precipitation of alumina hydrate from a slurry resulting from introduction of recycled alumina trihydrate seed into an aluminate liquor, in which particle size quality of alumina hydrate produced in the circuit and circulating in feed tanks is monitored, comprising the steps of: a) a calibration step including: a1) measuring, versus time, of: percent of alumina hydrate particles circulating in the feed tanks in the circuit that are finer than X2 μm; and percent of alumina hydrate particles circulating in the feed tanks in the circuit that are finer than X1 μm; where X1 and X2 are predetermined particle sizes in microns and X1 is smaller than X2; and a2) determining a relationship R between percent finer than X1 and later changes in percent finer than X2 μm, and defining upper and lower trigger thresholds of percent finer than X1 μm which correspond to maximum permissible variations in percent finer than X2 μm; and b) controlling the circuit, comprising measuring percent finer than X2 μm and forming a correlation between percent finer than X2 μm and the particle size of hydrate produced by the circuit, measuring percent finer than X1 μm and updating the relationship R, and causing corrective action to the slurry at the beginning of precipitation when the measured value of percent finer than X1 μm reaches an updated trigger threshold, to bring the percent finer than X2 μm within the maximum permissible variation.
 2. Process according to claim 1, wherein said corrective action includes modifying amount of solid in the slurry at the beginning of the precipitation.
 3. Process according to claim 2, wherein the modifying comprises varying amounts of pregnant aluminate liquor fed to a first agglomeration tank and a first feed tank.
 4. Process according to claim 1, wherein X2 is greater than 40 μm and X1 is less than 20 μm.
 5. Process according to claim 1, wherein the measurements of percent finer than X1 μm and percent finer than X2 μm are made on a slurry at the end of crystal growth phase.
 6. Process according to claim 1, wherein pregnant aluminate liquor feeding a first agglomeration tank in the circuit has a caustic content less than or equal to 160 g of Na₂O/liter.
 7. Process according to claim 1, wherein said calibration step comprises: 1) daily measuring percent finer than X1 μm in the slurry at a predetermined point in the circuit, which is used to produce a first particle size vs. time diagram represented by a curve Y=%<X1(t); 2) daily measuring percent finer than X2 μm in the slurry at a predetermined point in the circuit, which is used to produce a second particle size vs. time diagram represented by a curve Y=%<X2(t) and in which X2>X1 is a value already known to be well correlated with the particle size of the hydrate produced; 3) creating of an empirical relation between the particle size vs. time diagrams, which characterizes the relation R as: R(%<X2(t), %<X1(t−τ))=0 where t is the time at which percent finer than X2 μm is measured and τ is a characteristic time interval estimated by observing an occurrence of a same accidental phenomenon on each curve %<X2(t) and %<X1(t−τ); and 4) defining a maximum threshold and minimum threshold of percent finer than X1 μm obtained from the relationship R and a maximum interval of the permissible variation of values of percent finer than X2 μm.
 8. Process according to claim 7, wherein said controlling comprises: 1) daily measuring percent finer than X1 μm in the slurry at a predetermined point in the circuit, in order to complete the first particle size time diagram represented by the curve Y=%<X1(t); 2) daily measuring percent finer than X2 μm in the slurry at a predetermined point in the circuit, in order to complete the first particle size time diagram represented by the curve Y=%<X2(t); 3) updating a relationship between curve Y=%<X1(t) and curve Y=%<X2(t) and the definition of trigger thresholds of percent finer than X1 μm; and 4) triggering of a corrective action to modify amount of solid in the slurry at the beginning of the precipitation when the measured value of percent finer than X1 μm reaches one of the thresholds defined in 3). 